Plasma chambers commonly are used to perform processes for fabricating electronic devices such as semiconductors, displays and solar cells. Such plasma fabrication processes include chemical vapor deposition of semiconductor, conductor or dielectric layers on the surface of a workpiece or etching of selected portions of such layers on the workpiece surface.
A plasma commonly is formed in a plasma chamber by coupling electrical power from an electrical power source to a gas so as to excite the gas to a plasma state. If the electrical power source is a microwave power source, the power commonly is transmitted through a microwave waveguide connected between the power source and the plasma chamber.
In some designs, the microwave waveguide terminates in a dielectric window in the wall of the plasma chamber. This type of design typically couples power only to the plasma adjacent the dielectric window in the chamber wall, relying on diffusion to extend the plasma to the center of the chamber. Consequently, a disadvantage of this type of design is that it is difficult to achieve good spatial uniformity of the plasma density in a large chamber because the plasma density tends to be lower near the center of the chamber than near the surrounding chamber wall.
Other proposed designs differ from the above by extending the microwave waveguide into the interior of the plasma chamber. The waveguide has a hollow interior enclosed by a wall. Either a portion or all of the waveguide wall within the plasma chamber consists of a dielectric window. (The remaining portion typically is metal.) Microwave power is coupled to the plasma by means of a guided wave along the surface of the dielectric window. The section of the waveguide having a dielectric window commonly is referred to as a “microwave applicator”.
In the latter type of design, the interior of the waveguide cannot be maintained at the high vacuum of the plasma chamber, or else a plasma would be formed inside the waveguide, in which case the waveguide would cease functioning as a waveguide because plasma is a conductor rather than a dielectric. To prevent such plasma from forming, the interior of the waveguide must be filled with air (or other gas) at atmospheric or near-atmospheric pressure. This requires the dielectric window to provide a vacuum-tight gas seal so as to sustain the pressure differential between the air at atmospheric pressure within the waveguide and the process gases at high vacuum (i.e., very low pressure) within the plasma chamber.
A disadvantage of this type of design is that it is difficult to scale up to plasma chambers for processing workpieces that are several meters wide because it is difficult and expensive to manufacture a dielectric wall that is several meters long, can sustain the aforesaid pressure differential over its entire surface, and can withstand thermal shock from the high temperatures within a plasma chamber.
Some of the foregoing designs suffer the additional shortcoming of transmitting microwave power through a dielectric window facing only one direction. This can impair the spatial uniformity of the plasma produced within the plasma chamber.